D-liver D-letter, D-sooner, D-better!
del·ti·ol·o·gy n. The collection and study of postcards.
Google-ing for old postcards brings up a mind-bogglingly wide assortment of links, just a few of which I'll list here.
OldPostcards.com offers over 3 million old postcards, grouped by topic, including Sumo Wrestling, Elephant Tusk and Animals Smoking.
Postcardman.net is another biggie. Cards covering Dead People, Leper Colonies and Washerwomen.
Three guesses what "Postcard Bill" Judnick collects and sells. He runs lotsofcards.com, lotsofstamps.com and the interesting lotsofcovers.com, a huge collection of envelopes, postmarks and other postal artifacts.
Deltiology.com isn't the prettiest of sites. He has background images that make most of his pages practically unreadable, so when you see the link to turn them off, you might want to do so. He does have a cool Old-Time Postmark Calendar, though. Every day of the year shows an old postmark from the corresponding date. The bad news is that the calendars are from 1995, left over from the original printing run. The good news is that they're still interesting to look at, and in 2006, the dates will match up again and the calendar will be useful once again! He's asking a mere $3 apiece, or 5 for $12.